Keratan sulphate chains from bovine articular cartilage were fully digested with keratanase from Pseudomonas sp. and the products were reduced with alkaline borohydride. The resultant fragments were fractionated on a Nucleosil 5SB column and the earliest eluting fucose-containing oligosaccharides were isolated. Structural analysis using 1H n.m.r. spectroscopy (600 MHz) showed the two least-charged species to have the following structure: GlcNAc(6S) beta 1-3Gal beta 1-4(Fuc alpha 1-3)GlcNAc(6S) beta 1- 3Gal beta 1-4GlcNAc(6S) beta 1-3Gal-ol and GlcNAc(6S) beta 1-3Gal beta 1- 4(Fuc alpha 1-3)GlcNAc(6S) beta 1-3Gal beta 1-4GlcNAc(6S) beta 1-6(Gal beta 1- 3)GalNAc-ol. Both galactoses adjacent to the fucosylated N-acetylglucosamine residue are unsulphated. Therefore, it can be deduced from these structures that the presence of fucose on N-acetylglucosamine residues in keratan sulphates protects both of the adjacent unsulphated galactose residues from keratanase cleavage. This result has implications for the interpretation of keratanase fingerprints, because in articular cartilage keratan sulphates the keratanase-resistant blocks are not solely those with fully sulphated galactose residues, but also include the fucosylated sequences, which have unsulphated galactoses. It is, therefore, not possible to estimate their galactose sulphation or the size of the fully sulphated disaccharide-repeat sequences from keratan sulphates that contain fucose.